The invention concerns a method for conditioning of a roll, in particular of a roll in a paper machine or in a paper finishing device, in which method the condition of the face/coating on the roll is monitored and the face/coating on the roll is cleaned and/or ground.
The invention also concerns a device for conditioning of a roll, in particular of a roll in a paper machine and/or in a paper finishing device, which device comprises means for monitoring the condition of the face/coating on the roll and for cleaning and/or grinding the face/coating.
As is known from the prior art, in paper machines and in finishing devices of paper machines, the rolls in said machines and devices are contaminated, worn and/or damaged during operation, which has the consequence that the properties of the paper that is produced deteriorate. This is why the rolls must be cleaned and/or conditioned from time to time, in which connection the machine has to be stopped for the time of cleaning/conditioning. In some cases, the condition of a roll has deteriorated to such an extent that the roll must be replaced, which is detrimental in view of the production and may result in a standstill in the production in the paper machine.
An unevenness in the roll face, irrespective of whether it arises from contamination or consists of some other exceptional feature in the geometry of the roll face, causes alterations in the profile of the paper web. One particular problem related to coated rolls are the so-called hot spots, i.e. local heating of the roll. Hot spots arise in roll faces at an elevated zone arising from particles of contamination or equivalent, as a result of deformation arising from a higher surface pressure in the nips between the rolls, which results in formation of hysteresis heat, which causes damage to the roll coating at this location. This effect is made particularly problematic by the fact that the progress of a hot spot is rapid, and a hot spot is difficult to locate. Causes of disturbance include debris coming from outside or fibres or other contaminations coming along with the paper web.
One method of monitoring the face of a roll is visual examination of the roll, for example, by means of a camera. By means of detectors that have been fitted stationarily in view of monitoring of the condition of the roll bearings, it is possible to detect disturbance that affects the balance of the roll and also a hot spot that has developed sufficiently far. On the contrary, minor unevenness, single particles of contamination or a hot spot that is in its initial stage in the roll face cannot be detected by means of such methods.
In some prior-art solutions, the operator observes the roll face and, when he notices a hot spot, he eliminates the hot spot manually by means of a doctor. Currently, when recycled stock is used abundantly, adhesive constituents (stickies) contained in said stock increase the problem of hot spots. At present, paper machines are often connected with on-line coating, and the coating material (paste) must be dried before calendering. Sometimes the paste remains moist, which also causes increased hot spot problems.
The coating on a roll can also be damaged by what is called barring, in which case the roll face is subjected to formation of damage in the coating, which causes vibration. Barring means that on the roll face, in the axial direction of the roll, wear traces arise, which extend across the entire length of the roll and which resemble a certain sort of grooves. The process of formation of barring has not been established fully, but it is one explanation that minor variations in the thickness of the web in the longitudinal direction of the web produce wear of the roll face when said variations are repeated at suitable intervals.
The face of a roll is also worn during operation, in which connection the roughness of the face is increased and the quality of the paper deteriorates, in which case the roll must be removed for grinding and replaced by a replacement roll. For example, the thermo rolls used in the prior-art calenders are worn and become rough, in which case the linear load in the calender must be increased, which again reduces the bulk, and/or it is necessary to raise the calendering temperature, which reduces the bulk and also increases the consumption of energy.
A roll can also be worn unevenly, in which case the profile of the paper varies, and the roll must be removed for grinding and replaced by a replacement roll.
In the manufacture of paper, by means of a what is called calender, which is placed after the paper machine or which is connected directly with the paper machine, the face of the paper is processed to make the paper suitable in particular for printing machines so that the paper is passed through one or several roll nips. In the prior art currently applied, the roll nip in the calender is, as a rule, formed between a soft roll and a hard roll. The hard roll is made of metal, for example steel, and its face has been polished. The soft roll has been coated with an elastic synthetic material, for example some polymer. The paper web is passed into said nip formed by the pair of rolls, in which nip the smooth face of the hard roll smooths and glazes the paper web, and the elastic face of the soft roll provides the paper web with uniform density. If it is desirable to glaze both faces of the paper, the machine must have two pairs of rolls, so that one hard roll glazes one face of the paper and the other hard roll the other face. The hard rolls are usually heated rolls. In the U.S. Pat. No. 5,033,373 and in the published EP Patent Application 0,728,867 A2, calenders made of pairs of rolls are described, and the U.S. Pat. No. 5,131,324 describes a group of rolls in which there are one soft roll and two hard rolls. A soft roll differs from a hard roll in the respect that, since the face of a soft roll is made of some resilient material, for example polymers, particular attention must be paid to its servicing. A disturbing factor that is in its initial stage ought to be detected and located immediately in order that it could be eliminated directly. For example, in a two-nip calender, in which there is one variable-crown roll and one thermo roll in each nip, two replacement rolls must be kept constantly on stock, which involves costs. On the other hand, renewed coating of coated rolls is very expensive, so that a coating that has been spoiled, for example, because of a hot spot is quite expensive.
Since, as is well known, the condition of rolls has been monitored mainly by the operator manually in view of contamination, deterioration of condition, damage, for example hot spots, and in view of similar problems as well as by monitoring the quality of the paper, a number of replacement rolls have been necessary, and it has been necessary to renew coatings, which is expensive. Further, the standstills required by cleaning and conditioning operations have increased the costs.
With the use of various doctors, it is possible to clean the face of a roll during operation. Grinding can also be applied. With respect to the prior art related to the present invention, reference can be made, for example, to the FI Patent Applications 971488 and 941620 as well as to the U.S. Pat. No. 5,394,653 and to the EP Patent Application 0,359,304. In the FI Patent Application 971488, a band grinding device for rolls and a method for the control of the band grinding device in the grinding of faces of variable-crown rolls are described. The device and the method known from said published patent application are, however, meant to be used after the roll has already been removed from the machine to a grinding machine, and it is an advantage of said invention that the time taken by the grinding has become shorter. In the FI Patent Application 941620, a method and an equipment in conditioning of the coating on a roll are described, wherein the face of the roll is conditioned periodically and on the site without removing the roll. This arrangement does, however, not involve monitoring in order to establish the condition of the roll, and the solution suggested in said patent application involves providing of a doctor of the roll to be conditioned with a separate grinding device, which is fitted in the actuators of the doctor and brought into contact with the face to be ground by means of the doctor.
This device permits grinding of the roll in its site of operation. The U.S. Pat. No. 5,394,653 and the published EP Patent Application 0,359,304 describe traversing superfinishing devices for grinding of rolls, but said devices do not involve monitoring of the condition of the roll.
Thus, the object of the present invention is to suggest a method and a device by whose means the roll in a paper machine can be kept constantly in good condition, in which case the level of quality of the paper remains at the desired level.
It is a particular object of the invention to suggest an arrangement which is carried out continuously and during operation and which does not require a standstill for conditioning and/or cleaning of the roll.
One object of the present invention is to provide a method by whose means any divergencies in the roll face can be detected at a stage as early as possible.
The method in accordance with the invention is mainly characterized in that, in the method, the cleanliness and/or condition of the roll is/are measured continuously during operation and, based on the measurement results, the roll face/coating is cleaned and/or ground so that the roll remains constantly in good condition and the level of quality of the paper remains at the desired level.
On the other hand, the device in accordance with the invention is mainly characterized in that the device consists of an assembly of devices which comprises a cleaning/conditioning unit, a control unit, and measurement units, and that the device operates continuously and can be operated during running.
In accordance with the invention, certain parameters are measured, after which the grinding/cleaning device is controlled based on the measurement results, and the roll is cleaned/ground into good condition. In accordance with the invention, the cleanliness and the condition of the roll are measure d by means of a suitable method of measurement continuously on-line. At the same tire, it is possible to measure the shape and the circularity of the form of the roll, for example, by means of contact-free laser detectors. Further, in accordance with the invention, in the control, if desired, data on the thickness profile of the paper and possible other measurement data representing the quality of the paper as well as measurement data illustrating the operation of different parts of the paper machine are taken into account.
The device in accordance with the invention favorably consists of a traversing multiple-function head, by whose means the cleaning/conditioning of the roll is carried out, and of a control unit. The traversing beam that is used for the multiple-function head is most appropriately a prefabricated commercially available beam, and the multiple-function head preferably consists of a grinding unit, of a vibration-measurement unit that contacts the roll face, and of a unit for measurement of the shape and the circularity of the form of the roll.
The device in accordance with the invention can be fitted on the holder of a doctor, and by means of the measurement device, damage in the face is searched for and, if desired, it is possible to monitor the shape of the roll. As an auxiliary device for the device in accordance with the present invention, it is possible to use a traversing superfinishing device of the sort described in said U.S. Pat. No. 5,394,653 and in said published EP Patent Application 0,359,304.
In accordance with the present invention, the same grinding unit is used for cleaning and grinding of the roll face. When the roll is just contaminated, grinding takes place with a low grinding pressure, in which connection the face of the roll is not worn to a substantial extent, but the impurities are removed. If an error of shape has been worn into the coating, the grinding pressure is varied automatically so that it is higher at a thicker portion of the roll and is reduced at a thinner portion, in which way the error of shape can be corrected. Likewise, the grinding pressure can be varied during one revolution of the roll, in which way an error in the circularity can be corrected. By means of the device in accordance with the invention, it is possible to remove material, for example, at a hot spot, in which way its progress can be stopped. The grinding parameters for the grinding unit are selected automatically on the basis of the measured signals and of the signals received from the control system of the paper machine Besides the grinding pressure, it is also possible to regulate the traversing speed and the relative speed between the grinding element and the face to be ground.
As is well known, beams in paper machines involve errors of shape, which affect the roll grinding device. In connection with the device in accordance with the present invention, it is possible to provide devices of measurement, by whose means the shape of the roll and possible hanging down are measured, and said deficiencies are corrected by means of programming so that, even if a beam were curved or hanging, possibly slanting or bent, the grinding of the roll is, however, carried out so that a straight roll is obtained. For example, a laser beam can be used for measurement of a bending of a first order in the roll.
In the method in accordance with the invention errors in the linear shape of the traversing beam and bending of the beam, etc. are compensated for by means of programming. It is possible to use, for example, a laser beam parallel to the roll in the calender as a reference line, in which case the mechanical constructions do not have to be massive or manufactured with unreasonable precision. Thus, the system in accordance with the present invention compensates for its own faults on the basis of the control.
Rolls are not always fully circular, which causes problems. In accordance with the present invention, for example, onto the blade of a doctor, one or several acceleration transducers can be attached to an arm, in which case it is possible to measure the shape of the roll with very high precision.
In accordance with the present invention, it is possible to use moistening or chemicals in order to enhance the detaching of impurities and the grinding. Any excessive moisture is removed by means of blowing, suction, or a combination of same.
When the condition of the roll is maintained in accordance with the invention, the desired quality of the paper is achieved constantly, and no standstills are needed for replacement of the roll, the requirement of replacement rolls is reduced, and the service life of the roll coating becomes longer. If desired, the present invention can also be applied during a standstill without removing the roll.
According to a preferred embodiment of the invention, the face of the roll is measured by means of a laser detector, and data concerning the thickness of the paper are collected both in the machine direction and in the cross direction of the machine, and this same device, on which the measurement devices are fitted, is also used for cleaning and grinding the roll based on the measurement data. In the device, a grinding band is replaced by a cleaning band, for example a very fine diamond band or some other band specifically developed for cleaning, and impurities are ground from the roll face at a low pressure, in which case the measure of the roll is not changed practically at all, when it has been noticed that the roll requires cleaning. When it is noticed that the roll has an error of shape in the cross direction of the machine, i.e. in the longitudinal direction of the roll, the roll is ground intensively so that the error of shape can be corrected. If xe2x80x9cangular shapexe2x80x9d is noticed in the roll, the grinding is varied during one revolution, in which case the errors on the roll can be corrected during one revolution. The device in accordance with the invention is operated during running.
In the arrangement in accordance with the invention, when it has been noticed, for example, by means of measurement that a hot spot has been formed, the hot spot is located by means of the measurement devices, and material is removed by means of the grinding head of the device exclusively from the area of the hot spot on the roll down to the plane of the roll face or, if necessary, even so that a recess is formed below said plane. The device in accordance with the invention operates continuously, and the measurement and cleaning/grinding are carried out during running. This is very important, because hot spots are formed very quickly, and there are just a few minutes of time to take action.
Thus, the arrangement in accordance with the present invention is intelligent, it examines and acts on the basis of the results of its examination, and any defects, vibrations and equivalent in the roll can be corrected. When the operation takes place in accordance with the invention, the roll is constantly in good condition, in which case the level of quality of the paper remains constantly at the desired level. The invention is very well suitable for use in connection with polymer rolls and thermo rolls. When the device in accordance with the present invention is used, the desired quality is achieved constantly with a minimum of work, and possible standstill times related to cleaning/conditioning of the roll remain considerably shorter than when arrangements known from the prior art are used.
In the method in accordance with the present invention, preferably an accurate displacement-measurement technology is used for detecting of the disturbance points. In the method, the vibration detector(s) has/have been fitted on a member that is in contact with the roll face. When the roll revolves, the point of discontinuity arrives at the location of the detector at regular intervals and produces a signal, and even minor unevenness in the roll face, i.e. changes in the geometry of the circumference of the roll, can be detected immediately.
It is an important advantage of a preferred arrangement of measurement employed in connection with the present invention that, to carry out the invention, it is possible to employ fully commercially available technology, and the necessary components have been produced in series, in which case application of the method is highly economical, as compared with other methods.
In the publication EP 0,728,867 A2 mentioned above, it is stated that the average roughness of the face of a soft roll does not exceed the value Ra 0.5 xcexcm, which gives an idea of how smooth the faces of soft rolls can be machined. In experiments that have been carried out, it has been noticed that by means of both embodiments of the invention, which will be described later, it is possible to notice defects of shape in a roll face whose divergence from the roll face is of an order of 1 xcexcm and even smaller. An equally good result cannot be achieved by means of any other mode of measurement.
Since the method is quick and its resolving power is very good, with mobile detectors this also permits a very short traversing time (xe2x89xa730 s). Even small defects and errors of shape can be detected quickly. Since the method is very rapid and accurate, barring of a face that is still at its initial stage can also be detected very early. The basic cause of barring is not yet fully known, but if and when it will be known in the course of time, the beginning of barring can be detected at a very early stage, and the progress of barring can be stopped or barring be eliminated completely by controlling the paper machine or the calendering process.
By means of the advantageous arrangement of measurement equipment in accordance with the present invention, the condition of the face of a roll can be monitored accurately in real time. The equipment comprises a contact member fitted against the face of the roll, on which member a displacement-measurement detector has been fitted, which is connected to a data transfer line in order to transfer the signal produced by the detector along the data transfer line to a recording unit included in the equipment.